U.S. Pat. No. 3,208,860 (Armistead et al.) provides a general discussion of the theoretical concepts underlying the phenomenon of photochromism in glass (formerly termed phototropism). It also outlines the practical considerations which must be observed in producing such glass. As illustrative of the phenomenon, the patent provides exemplary compositions of inorganic silicate glasses which contain submicroscopic crystals of at least one of the silver halides, viz., silver chloride, silver bromide, or silver iodide. When subjected to actinic radiation, customarily ultraviolet radiation, the crystals become darker in color, thereby decreasing the optical transmission of the glass. However, when the actinic radiation is removed, the crystals return to their original state and the optical transmission of the glass is restored.
It is postulated that the actinic radiation reacts on the crystals dispersed within the glassy matrix to alter the absorptive characteristics of the crystals to visible light. Unlike the normal photographic film, glass is inert and impermeable. Therefore, when the impingement of actinic radiation on the glass is halted, the crystals are permitted to return to their original state, since the reaction products resulting from the exposure of the crystals to actinic radiation cannot diffuse away from the site of the reaction. This circumstance allows the transmission of such glass to visible light to be reversibly varied ad infinitum.
U.S. Pat. No. 3,325,299 (Araugjo) discloses the production of photochromic glass from inorganic glass compositions comprising copper, cadmium, and a halogen selected from the group consisting of chlorine, bromine, and iodine as essential components. The photochromically-active constituent of these glasses is a copper-cadmium halide phase which darkens on exposure to 300-550 nm. radiation and fades in the absence thereof.
Glasses of the type described in U.S. Pat. No. 3,325,299 offer certain advantages over photochromic glasses of the silver halide type, principally with respect to batch cost and photochromic behavior. The darkening of these glasses is more proportional to the intensity of incident light, and less dependent upon temperature, than is the case with silver halide glasses. Also, these glasses tend to absorb actinic radiation strongly. Hence they darken only in a relatively thin surface layer of the exposed glass, and exhibit high darkened optical density even in thin cross-section.
Unfortunately, photochromic glasses of the copper-cadmium halide type also suffer certain disadvantages, especially with respect to the color of the glass. Thus, these glasses in general exhibit only greenish coloration in the darkened state.
U.S. Pat. No. 4,076,544 (Kerko et al.) discloses photochromic glasses similar to those disclosed in U.S. Pat. No. 3,325,299, but containing a photochromic phase which includes copper, cadmium, silver and at least one halogen selected from the group consisting of chlorine, bromine and iodine. These glasses are susceptible to optical bleaching and retain rapid thermal fading and surface darkening characteristics. However, when darkened, they tend to produce purple to reddish-brown colors.
The darkened color of a photochromic glass may not be of particular significance in apparatus components, such as display panels or memory devices. The situation is quite different in ophthalmic use for example. Here, a neutral or inconspicuous color may be greatly preferred. Hence, neither the green colors provided by the copper-cadmium halide phase of U.S. Pat. No. 3,325,299, nor the purple or reddish brown colors provided by the silver-copper-cadmium phase of U.S. Pat. No. 4,076,544, are considered desirable for such purpose.